This invention relates to a xerographic developing device of an electrophotographic image forming apparatus such as a copier for forming a visible image on a transfer sheet by attaching toner to an electrostatic latent image on a photoreceptor and more particularly to such a developing device provided with a removably attached cartridge such that deteriorated developing agent can be economically and efficiently replaced with a fresh supply.
Among developing devices of this type, there are those for using a single-component developing agent and those for using a two-component developing agent comprised of toner particles which are thermoplastic resin particles and carrier particles of a magnetic substance. A two-component developing agent is stirred inside the developing device and toner is electrostatically adsorbed on carrier surfaces. A magnet roller comprised of a cylindrical sleeve and magnets positioned inside this sleeve is disposed inside the developing device opposite to the copier's photoreceptor. As the sleeve rotates, the carrier particles with toner attached thereon are magnetically adsorbed by the rotating magnetic field which results and are delivered to the surface of the photoreceptor.
Since the electrostatic image formed on the photoreceptor surface is of the polarity opposite to that of the charge on the toner, toner leaves the carrier and is transferred to be attached to the electrostatic latent image on the photoreceptor. In the meantime, the carrier remains on the sleeve and returns to the interior of the developing device. Thus, it is only the toner that is consumed by the developing processes. As for the carrier which is used over and over again, its charging characteristics deteriorate after a repeated use in several thousand to several hundred thousand image forming processes. With a deteriorated carrier, toner cannot be charged to a sufficient voltage and insufficiently charged toner may be attached onto areas on the photoreceptor not having any electrostatic image. This tends to adversely affect the quality of the transferred image. Moreover, if electrostatic adsorption becomes too weak, toner particles will easily fall off from the developing device or the surface of the photoreceptor, and scatter around inside the copier housing. It is therefore necessary to replace the developing agent if the charging characteristics of its carrier have deteriorated.
It is not a simple task to replace developing agent, however, because the carrier, which is a magnetic substance, does not easily come off the sleeve surface of the magnet roller. Moreover, the stirrer for the developing agent is of a complicated shape and makes it difficult to remove the developing agent from the interior of the developing device. It usually requires the service of a trained specialist for the job.
In view of the above, several devices and methods for collecting developing agent have been proposed in order to simplify the replacement of developing agent. Japanese Patent Publication Tokkai No. 54-111354 disclosed a developing tank with a collecting vessel disposed in the circulating route therein and use of a magnet to collect the residual agent at the bottom. Japanese Utility Model Jikko No. 57-55623 disclosed a method of moving a magnetic piece inside the developing tank. Japanese Patent Publication Tokkai No. 60-239775 disclosed a developing tank with a concave bottom surface with an outlet for collecting developing agent. Japanese Patent Publication No. 60-107057 disclosed a developing tank of which a shutter at the bottom along the entire length of its circulating route is openable such that developing agent can be collected into a container removably attached to the bottom. Japanese Patent Publication Tokkai No. 60-140368 disclosed a developing tank separably provided with a device for supplying developing agent and a device for removing residual developing agent. Japanese Patent Publication Tokkai No. 60-153066 disclosed a developing tank having a toner cartridge at the top E and a collection cartridge at the bottom.
A common disadvantage of all these prior art technologies is that a container dedicated for the collection of developing agent must be provided separately from the developing device. This makes the work complicated and troublesome. In view of the above, there have also been copiers of which the developing device is intended to be discarded entirely together with deteriorated developing agent remaining inside. This, however, is economically disadvantageous because relatively expensive, otherwise more durable components contained within the developing tank such as a magnet roller and a toner sensor must also be discarded and hence replaced.
A still further problem to be considered has been that the user could not tell whether the developing agent inside a cartridge which has been set is unused or already deteriorated. The user may therefore be using old developing agent, thinking it is new. If a copy of poor image quality is thereby obtained, the user cannot readily ascertain what has caused the poor result.